The present invention relates to a seat belt retractor.
A seat belt retractor returns a seat belt to its fully retracted position after use. The seat belt is wound onto a winding shaft until it comes into the vicinity of the vehicle""s B-pillar in the fully retracted position.
It is well known to use a motive spring, for example in the form of a spiral spring, to rotate the winding shaft. One end of the motive spring is connected in a non-rotatable manner to the winding shaft and the other end of the motive spring rests on a fixed point, for example the casing of the retractor fixed to the vehicle structure. While the seatbelt is in use the spiral spring constantly applies a force to the seatbelt webbing in the retracting direction. While handling the seatbelt, the vehicle occupant will feel a constant pull on the seatbelt webbing, and when the vehicle occupant releases the unbuckled seatbelt, the seatbelt will return to its fully retracted position. While the seatbelt is in use the vehicle occupant may experience some discomfort because the seatbelt could be too snug or tight.
A so-called comfort retractor is known, for example from published German patent application 41 12 620, to overcome the above-described problems. An electric motor is provided for the winding process in addition to the motive spring. The motive spring is held in a spring casing, the inner end of the motive spring in turn being connected in a non-rotatable manner to the winding shaft, but the outer end acting on the internal periphery of the spring casing. External teeth that engage a pinion of the motor are provided on the outer periphery of the spring casing. If the belt webbing is unwound or pulled out by hand, the electric motor is supplied with current and moves the spring casing in the direction of rotation increasing the bias of the motive spring until a blocking device responds and prevents further rotation after a predetermined number of rotations of the motor. In this biased state the motive spring the spring casing is held by the stopped electric motor until the tongue is inserted into the buckle and a switch is opened. The winding spring then turns the spring casing back again, the force of application of the belt webbing being reduced so it is no longer unpleasant for the vehicle occupant.
If the tongue is pulled out of the buckle, the above-mentioned switch closes again and restarts the electric motor. The winding spring is then biased again by rotation of the spring casing to such an extent that it reliably retracts and re-winds the belt webbing.
The foregoing solution from the prior art is unsatisfactory in so far as a certain amount of time elapses after release of the tongue from the buckle, until the actual winding process that is brought about by the motive spring. This delay is due to the motor initially having to bring the spring casing back into a position in which the motive spring is tensioned in such a way that it can perform a winding process.
Owing to the bridging device it is possible, even with a motive spring having only low, or no, spring tension to begin the winding process immediately after release of the tongue from the buckle, by using the force of the motor drive unit for the winding process. In contrast to the aforementioned state of the art, the motive spring is not used for the winding process, but the drive unit, in particular the motor contained therein, which is preferably an electric motor, in the retractor according to the invention.
If the bridging device is to act directly between drive unit and winding shaft the drive unit would have to be arranged in the vicinity of the winding shaft, and this necessitates an additional configuration to enable the drive unit to be used for relaxing the motive spring when the tongue is inserted into the buckle. In user-friendly retractors, the motive spring is frequently accommodated in a rotatable spring casing to which the torque of the drive unit can be transmitted. In such a case it is advantageous for a compact construction of the retractor if the bridging device, on the one hand, is connected in a non-rotatable manner to the spring casing and, on the other hand, can be brought into rotational engagement with the winding shaft for transmitting a torque of the drive unit.
In principle, the bridging device can have quite different designs. A particularly advantageous solution can be achieved if the bridging device is a centrifugal clutch (switched according to the number of revolutions). This has the advantage that the bridging device is only xe2x80x9cconnectedxe2x80x9d when a specific, predetermined speed is exceeded. The drive unit can be used both during the relaxation process of the motive spring when the seat belt is applied and during the winding process. If the speed of the drive unit is lower than the aforementioned limit value, the bridging device remains xe2x80x9cdisconnectedxe2x80x9d and the drive unit can rotate the spring casing into the load releasing direction of the motive spring. If the speed of the drive unit exceeds the limit value, the bridging device is automatically switched and the drive unit drives the winding shaft for winding the belt webbing.
In the switched state the centrifugal clutch can produce an interlocking fit or frictional contact for torque transmission between the drive unit and the winding shaft, frictional contact being preferred.
The construction of the centrifugal clutch can, in turn, be quite different. It is particularly advantageous if the centrifugal clutch comprises at least one spring element that changes its shape elastically under the influence of the torque and is formed by at least one damping element. The damping element has bringing about speed-dependent switching in such a way that the centrifugal clutch remains disconnected at a speed of the drive unit below a predetermined limit value but the centrifugal clutch produces frictional contact with the winding shaft when the limit value is exceeded. The spring element can annularly surround the winding shaft.
There is provided in accordance with the invention a seat belt retractor comprising a winding shaft for winding a seat belt, a motive spring for biasing the winding shaft in the winding direction of the seat belt and a drive unit for the at least partial relaxation of the motive spring when the seat belt is at least partially unwound, wherein a bridging device is provided by means of which the winding shaft can be driven directly by the drive unit during a winding process of the seat belt.